1. Field of the Invention
The present invention relates to a camera shutter having a pair of sector drive rings for opening and closing sectors, and a pair of electromagnetic units for releasing the sector drive rings to allow the sectors to be opened and closed.
2. Description of the Prior Art
Camera shutters having two sector drive rings and two electromagnetic units of the type described above are known in the art as described in Japanese Patent publication No. 35-14275. With such a camera shutter, both of the sector drive rings are held in a set position for keeping the sectors in the closed position under attractive forces from the electromagnetic units against the resiliency of drive springs. When one of the sector drive rings is turned under the force of the drive spring upon de-energization of the corresponding electromagnetic unit, the sectors are opened. Upon elapse of a prescribed interval of time, the other electromagnetic unit is de-energized to permit the other sector drive ring to turn in the same direction under the resiliency of the other drive ring for thereby closing the sectors, whereupon desired exposure is completed. Therefore, the sectors can repeatedly be opened and closed by angularly moving the sector drive rings as they remain relatively positioned to the set position against the bias of the drive springs while the sectors are being closed, until the sector drive rings are attracted by the electromagnetic units, and thereafter by de-energizing the electromagnetic units to release the sector drive rings.
The camera shutters of the above construction are advantageous in that the sectors can be closed before they are fully opened by releasing the sector closing drive ring while the sectors are being opened by the sector opening drive ring as it is actuated. Therefore, the sectors can be used also as a diaphgram for enabling high-speed exposure in large-size shutters.
For such high-speed exposure, the sector closing drive ring starts to operate while the sectors are on their way to the open position, that is, while the sector opening drive ring is being actuated. Since the sectors are opened and closed under conditions detemined by the relative speed of movements of the drive rings, the movement of the sectors is largely affected by frictional resistances that the drive rings are subjected to and also by other operational fluctuations. This renders the sector opening curve and hence the exposure time less stable, with the result that the accuracy of operation of the shutter is lowered.